1. Field of the Invention
The present invention relates to a ceramic capacitor and a method for the preparation of the ceramic capacitor and, to more particularly, to a ceramic capacitor of a mono-layer structure or of a multi-layer structure, having one or more than one dielectric layer composed of a dielectric ceramic composition containing BaTiO.sub.3 as a major component and two or more than two internal electrodes so disposed as to interpose the dielectric layer.
2. Description of the Related Art
Ceramic capacitors of a multi-layer structure are generally formed by alternately laminating a non-sintered ceramic sheets for forming dielectric layers and conductive paste films and baking the laminated dielectric layers and conductive paste films at temperatures ranging from 1,300.degree. C. to 1,600.degree. C. in oxidative atmosphere. Hence, expensive metals as are not oxidized upon baking in oxidative atmosphere, such as platinum or palladium, are employed as material for the internal electrode, thereby making costs of ceramic capacitors expensive.
In order to reduce the costs of the ceramic capacitors, it is considered to use a base metal, such as Ni or the like, as the material for the internal electrodes and to bake the base metal in reductive atmosphere to prevent the oxidation of the base metal. It is to be noted, however, that desired dielectric ceramic compositions having sufficiently high electrical characteristics cannot be produced by baking material of the dielectric ceramic compositions so far known in reductive atmosphere.
For example, Japanese Patent Laid-open Publication (kokai) No. 61-275,164 discloses dielectric ceramic compositions having a high dielectric constant yet a small rate of change of temperatures of capacitance which can be produced by baking powder of raw material obtainable by adding Nb.sub.2 O.sub.5 and Co.sub.2 O.sub.3 to BaTiO.sub.3, in oxidative atmosphere. It is to be noted, however, that the baking of such powder of the raw material in such reductive atmosphere cannot produce the dielectric ceramic composition having desired electrical characteristics because the baked body may become readily converted into semi-conductors.
Japanese Patent Publication (kokoku) Nos. 61-14,607-14,611 and 62-1,595-1,596 propose dielectric ceramic compositions, obtainable by baking in reductive atmosphere, having a high dielectric constant yet a small rate of change in temperatures of the capacitance. Further, our Japanese Patent Application Nos. 1-311,094-98, 2-314,754-56, 1-315,981-83, 2-16,345, 2-76,761-68 and 2-76,773-776 propose improvements in various electric characteristics of the dielectric ceramic compositions.
The dielectric ceramic compositions as proposed by those patent applications as described hereinabove contain, as a first basic component, a component as represented by (Ba.sub.k-x M.sub.x)O.sub.k (Ti.sub.1-y R.sub.y)O.sub.2-y/2. As a second basic component, there may optionally be employed a component as represented by CaZrO.sub.3 or BaZrO.sub.3. Further, as a first additive component, there may be employed Li.sub.2 O-SiO.sub.2 -MeO or B.sub.2 O.sub.3 -SiO.sub.2 -MeO, while at least one of Cr.sub.2 O.sub.3 and Al.sub.2 O.sub.3 may optionally be employed as a second additive component.
Recently, however, very strong demands have been made to minimize laminated ceramic capacitors as electronic circuit becomes highly dense. For making laminated ceramic capacitors compact, there are known many processes for reducing the thickness of the dielectric layers and for increasing the number of layers to be laminated. If the dielectric layer becomes too thin, however, the direct current breakdown voltage may deviate and reliability in the withstand voltage of the ceramic capacitor cannot be maintained.
For instance, for laminated ceramic capacitors requiring for characteristic X7R (characteristic B), reliability in the direct current breakdown voltage particularly at the highest temperature as high as 125.degree. C. places particular significance. However, reducing the thickness of the dielectric ceramic compositions disclosed in the aforesaid patent applications to deviation in the direct current breakdown voltage at 125.degree. C. and makes it impossible to maintain the reliability in the withstand voltage at that temperature.